The field of the invention is nuclear magnetic resonance imaging ("MRI") and magnetic resonance angiography ("MRA"). More particularly, the invention relates to local coils used to receive NMR signals from specific regions of the human anatomy.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .gamma. of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.O), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M, is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t, which is rotating, or spinning, in the xy plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B.sub.1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance phenomena is exploited.
Commercially available NMR imaging systems utilize a coil which surrounds the entire patient for receiving the NMR signals produced by excited spins. Such "whole body coils" have the advantage that they are substantially uniformly sensitive to NMR signals produced over a large region to be imaged. However, because of their large size, whole body coils are relatively insensitive to NMR signals emanating from specific regions and they have a relatively large inductance. In addition, whole body coils receive noise from regions in the body outside the imaging region, thus further lowering the SNR ratio. Where more sensitivity is desired from a particular region of the human anatomy, it is common practice to employ a small "local" or "surface" coil which is shaped to provide increased sensitivity over the particular region of interest. Local coils designed for the knee, wrist, shoulder, neck or head, for example, have been designed. While such local coils provide images of these regions with an improved signal to noise ratio ("SNR") their fields of view are very limited. They are not appropriate, therefore, for imaging an elongated region such as the leg without repeatedly interrupting the scan and repositioning the local coil over a different segment of the leg.
As described in U.S. Pat. Nos. 4,721,913 and 4,825,162, it is possible to employ an array of local coils in such a way that the field of view of the combined signals they receive is substantially increased without significantly reducing their sensitivity. The fields of view of each coil in these local coil arrays is different, but they overlap to provide a substantially constant sensitivity over their combined fields of view. The high SNR of each local coil in the array is maintained by arranging them such that the mutual inductance between them is minimized.
When performing an NMR scan of the legs, the high SNR of a local coil is required. However, such scans must also gather NMR data from the entire length of the leg in order to completely image the vasculature. Currently, local coils such as that described in U.S. Pat. No. 4,724,389 are used for this purpose, and they must be moved many times during the scan to acquire NMR data from the entire leg. This is very time consuming and increases the cost of such scans very significantly. It is also very inconvenient for the patient.